This project is a direct outgrowth of my long-term study of the chromosome pattern of leukemic cells obtained from patients who had previously been exposed to radiation and/or chemotherapy. Initially, we concentrated on the common abnormalities, namely deletions of chromosomes 5 and/or 7. This remains the focus of Projects 2 and 3. As we studied more patients, it became apparent that another group of therapy-related acute leukemia patients has translocations or deletions of chromosome 11, band 11q23; the most common abnormality is a t(9;11)(p22;q23). These patients often have acute monoblastic or myelomonocytic leukemia which is also associated with rearrangements of 11q23 in patients with de novo leukemia. It is of interest that a substantial fraction of the treatment-related leukemic patients have received epipodophyllotoxins, either etoposide (VP16) or teniposide (VM26). These drugs are known to inhibit topoisomerase II (Topo II) and to cause chromosome breaks when added to cultured cells. Topo II is thought to be a major protein component of the nuclear matrix and both Topo II cleavage sites and matrix attachment regions have been located adjacent to structural chromosome rearrangements. I propose to use cosmid probes and yeast artificial chromosomes (YACs) from 11q23, labeled with various fluorochromes, to identify the breakpoint junction in leukemic cells and cell lines and hybrid somatic cells derived from leukemic patients with an abnormality of 11q23. We have shown that one YAC is split in four different translocations, t(4;11), t(6;11), t(9;11), and t(11;19). It is not split in two other translocations. When the breakpoint junction has been detected, the same probes or subclones from the probe will be used to detect rearrangements with standard and pulsed-field gel electrophoresis. Once a junction has been cloned, the probe from 11q23 will be used to determine whether the breakpoint is similar in cells from patients with treatment-related and de novo leukemia. DNA probes will be used to map Topo II cleavage sites and matrix attachment regions relative to the DNA sequences at the breakpoint junction. This analysis should provide data on the nature of the gene(s) located in 11q23 that leads to preferential involvement in monoblastic leukemia. It may also provide insights as to the structure of this gene relative to its apparent sensitivity to rearrangement on exposure to Topo II inhibitors.